There is a big difference between brother/sister act and actual brother and sister. One could say that Hawkeye and Black Widow from the Avengers was a brother/sister act in some ways. I think it'll just be two characters who are like bro and sis. Personally I wouldn't like to see Scarlet Witch and Quicksilver yet. I mean it'd be great but their backstory really adds to the characters motives. We'll have to wait and see!